Computed tomography (CT) is a widespread diagnostic imaging method which measures the x-ray attenuation coefficient of matter. This x-ray attenuation coefficient is depicted in terms of Hounsefield Units (HU). During a CT scan, a collimated X-ray beam is directed on the patient and the attenuated remnant radiation is measured by a detector whose response is transmitted to a computer. The computer considers the location of the patient and the spatial relationship of the x-ray beam to the region of interest. The computer analyzes the signal from the detector so that a visual image can be reconstructed and displayed on a monitor. The image can then be viewed or stored for later evaluation.
Hounsefields Units reflect the relative absorption of CT x-rays by matter, the absorption being related to the atomic number, electron density, physical thickness of that matter, and the energy spectrum of the x-rays. Because of the similarity in electron density of various tissues in the body, CT scans sometimes result in poor imaging. In an attempt to obtain better results in such circumstances, a contrast agent, such as iodine, can be injected in the patent's blood stream to change the relative radio-density of the tissues, and improve overall diagnostic efficacy.
When using a contrast agent, it is extremely important to coordinate the time of the scan with the time of greatest levels of contrast in the region of interest, in some instances with respect to a threshold value. Because the contrast agent is injected into the blood stream, many physiological factors can affect the start time and duration of a sufficient level of contrast in the region of interest. For example, because the cardiovascular system provides the means for circulation of contrast agent throughout the body after it is injected into the blood stream, a patient's cardiac output can have a significant effect on the distribution of the contrast agent as well as the time taken for the contrast agent to reach a particular organ or vessel.
Current understanding of intravenous contrast enhancement is further complicated by multiple interacting factors including contrast agent type, volume and concentration, injection technique, catheter size and site, scanning technique, patient characteristics and tissue characteristics. Of these factors, all of which have influence on contrast enhancement, the variables which cannot be controlled are those related to the patient. These include age, gender, weight, height, cardiovascular status, renal function and other disease status. In the past ten years, many clinical studies testing various intravascular contrast agent and injection protocols have been reported. However, in many respects, contrast enhancement still relies heavily on the experience and intuition of the physician rather than rigorous, quantitative analysis of the mechanism of contrast enhancement.